1. Field of the Invention
The present invention relates to an organic EL (Electroluminescence) device and a method of manufacturing the organic EL device, and more particularly, relates to the organic EL device including metallic oxide at an interface between an organic layer including an organic luminescent layer at least and a cathode and the method of manufacturing the organic EL device.
The present application claims priority of Japanese Patent Application No. 2001-174444 filed on Jun. 8, 2001, which is hereby incorporated by reference.
2. Description of Related Art
As one of EL elements used for a display device for an information apparatus or a like, an organic EL device is developed. FIG. 12 is a sectional view showing a structure of a conventional well-known organic EL device. The organic EL device, as shown in FIG. 12, is provided with a transparent insulation substrate 51 made of a glass substrate or a like, an anode (lower electrode) 52 made of transparent conductive material such as an ITO (Indium Tin Oxide) formed on the transparent insulation substrate 51, a hole transport layer 53 formed on the anode 52, an organic luminescent layer 54 formed on the hole transport layer 53, a cathode (upper electrode) 55 made of AlLi (Aluminum Lithium) or a like formed on the organic luminescent layer 54, and a cap 57 attached to the transparent insulation substrate 51 on which an element main portion including the anode 52, the hole transport layer 53, the organic luminescent layer 54 and the cathode 55 is formed so as to cover the element main portion via seal resin 56.
As the seal resin 56, for example, UV (Ultra-Violet) curable resin is used, and is cured by irradiating light including UV rays to the seal resin 56 from a light source so as to seal the EL element.
In the above-mentioned organic EL device, since there are many cases in that the interface between the organic luminescent layer 54 and the cathode 55 is incomplete, an unstable defect exists from the first. The defect indicates that an impurity level caused by a grid defect or a like is formed at a position at which an interface level must be formed. With existence of the defect, a path generates in addition to a normal carrier path, and then a leak current increases. Also, there is a possibility in that the cathode 55 and the anode 52 become short-circuited. Therefore, a characteristic of the organic EL device becomes unstable, and a high rectification ratio can not be obtained. As a result, when a simple matrix drive is performed, a pixel short-circuit and a cross talk occur.
Now, conventionally, there is another well-known organic EL device in which an oxide layer is formed at an interface between an organic luminescent layer and a cathode so as to make a characteristic stable. For example, Japanese Patent Application Laid-open No. Hei 9-245968 discloses such the organic EL device. The organic EL device, as shown in FIG. 13, is provided with a glass substrate 61, an anode 62 made of ITO formed on the glass substrate 61, a hole transport material layer 63 made of an organic material formed on the anode 62, an electron transport material layer 64 made of an organic material formed on the hole transport material layer 63, and a cathode 65 formed on the electron transport material layer 64. An oxide layer 66 (or a hydroxide layer) is formed at an interface between the electron transport material layer 64 and the cathode 65.
As a material for the cathode 65, a univalent metal such as Li (lithium), Na (sodium) and K (potassium), a bivalent metal such as Ca (calcium), and a trivalent metal such as Al (aluminum) are used. Concretely, when a film of the material of the cathode 65 is formed, the oxide layer 66 is formed by making a concentration of oxygen molecules or hydrogen molecules in an atmosphere higher than that of ordinary film formation.
With this arrangement, a work function of metals or ionization potential of organic material at the interface between the electron transport material layer 64 which is an organic luminescent layer and the cathode 65 is lowered, whereby a low voltage drive is possible. As a result, it is possible to obtain the organic EL device which operates stably.
However, in the organic EL device disclosed in Japanese Patent Application Laid-open No. Hei 9-245968, since only the oxide layer is formed at the interface between the organic luminescent layer 64 and the cathode 65, there is a problem in that it is difficult to obtain a high rectification ratio.
In other words, in Japanese Patent Application Laid-open No. Hei 9-245968, when the film of the material of the cathode 65 is formed in manufacturing the organic EL device, the oxide layer 66 is formed by making a concentration of oxygen molecules or hydrogen molecules in the atmosphere higher than that of ordinary film formation. Therefore, since it is difficult to control the concentration of oxygen molecules or hydrogen molecules in a desirable range, a yield of manufacturing the organic EL device gets worse, a throughput lowers, and an enough high rectification ratio can not be obtained. Further, the oxide layer 66 (or the hydroxide layer) is an insulation layer, and therefore, when the oxide layer 66 (or the hydroxide layer) is formed evenly, there are problems in that a drive voltage is high and a luminescent efficiency lowers.
In view of the above, it is an object of the present invention to provide an organic EL device and a method of the organic EL device capable of obtaining a high rectification ratio with a low drive voltage and capable of making a luminescent efficiency high.
According to a first aspect of the present invention, there is provided an organic EL device in which an anode made of a transparent electrode, an organic layer including an organic luminescent layer at least, and a cathode sequentially stacked on a transparent insulation substrate, the cathode including a metallic oxide in an interface with the organic layer wherein; the metallic oxide is formed so as to have a concentration gradient in which a concentration becomes high at a side of the interface with the organic layer.
In the foregoing, a preferable mode is one wherein the cathode includes a first cathode and a second cathode and the first cathode includes the metallic oxide in the interface with the organic layer.
Also, a preferable mode is one wherein the cathode includes a plurality of layers and a cathode among the plurality of layers includes the metallic oxide in the interface with the organic layer.
Also, a preferable mode is one wherein the concentration gradient of the metallic oxide varies curvedly along a direction of a film thickness of the cathode.
Also, a preferable mode is one wherein the concentration gradient of the metallic oxide varies linearly along the direction of the film thickness of the cathode.
Furthermore, a preferable mode is one wherein a concentration of the metallic oxide becomes 0 (zero) in the film thickness of the cathode forming the interface with the organic layer.
According to a second aspect of the present invention, there is provided a method of manufacturing an organic EL device in which an anode made of a transparent electrode, an organic layer including an organic luminescent layer at least, and a cathode sequentially stacked on a transparent insulation substrate, the cathode including metallic oxide in an interface with the organic layer wherein;
the transparent insulation substrate on which the anode and the organic layer are sequentially stacked is put into a vacuum evaporation apparatus, and a first metal capable of forming the metallic oxide and a second metal for forming the cathode are co-evaporated in a manner that the metallic oxide having a concentration gradient in which a concentration becomes high at a side of the interface with the organic layer is formed.
According to a third aspect of the present invention, there is provided a method of manufacturing an organic EL device in which an anode made of a transparent electrode, an organic layer including an organic luminescent layer at least, and a cathode sequentially stacked on a transparent insulation substrate, the cathode including metallic oxide in an interface with the organic layer, the method including:
a step of laminating the anode and the organic layer on the transparent insulation substrate sequentially;
a step of putting the transparent insulation substrate into in a vacuum evaporation apparatus to which a first metal capable of forming the metallic oxide and a second metal for forming the cathode are supplied; and
a step of forming the metallic oxide having a concentration gradient in which a concentration becomes high at a side of the interface with the organic layer by co-evaporating the first metal and the second metal.
In the foregoing, a preferable mode is one wherein a surface of the first metal is previously oxidized.
Also, a preferable mode is one wherein co-evaporation is performed while introducing oxygen into the vacuum evaporation device, and is performed with control in a manner that the oxygen in the vacuum evaporation apparatus decreases gradually from a start of the co-evaporation.
Also, a preferable mode is one wherein the first metal is made of Li or Mg, and the second metal is made of Al or Mg.
With these configurations, since an interface between a cathode and an organic layer including an organic luminescent layer at least includes metallic oxide, and the metallic oxide is formed to have a concentration gradient in which a concentration becomes high at a side of the interface with the organic layer, defects existing in the interface between the organic layer and the cathode are embedded by the metallic oxide, and the interface is formed completely.
Also, when the organic EL device is manufactured, a metal capable of forming metallic oxide by surface oxide under an atmosphere including oxygen such as an air atmosphere and a cathode formation metal are used as evaporation sources, and both of the metals are co-evaporated. Therefore, it is possible to form metallic oxide having a concentration gradient in which a concentration becomes high at the side of the interface with the organic layer.
Therefore, a rectification ratio is improved by preventing short-circuit while applying backward bias application to an element and by saving a leak current. Also, when passive matrix driving is performed, it is possible to prevent pixel short-circuit and cross-talk.
Further, by providing the concentration gradient, it is possible to make the driving voltage low and to make the luminescent efficiency high.